Set oven to 200C. In a wok or pan, caramelise leeks and apples. In a large bowl put leek and apple mix, and stir in rosemary, vinegar, creme Fraiche and cheese. Roll out pastry to fit a 35cm x 25cm (or whatever fits) and fit it in the pie dish. Tip the mix into the pastry and bake for 30-45 minutes, or til simmering at the sides.